There is a need for an improved magnetic compass system especially adapted for use in vehicles, such as land vehicles and small boats, for the purpose of general orientation and for navigational purposes. Magnetic compasses for automotive vehicles are known in various forms. Typically, such compasses are of the type which utilize a compass card which carries the compass magnet and a direction indicator is provided thereon within the view of the vehicle driver. It is often desirable to install the compass per se, i.e. the magnetic field sensor and the compass display at separate locations in the vehicle.
A magnetic compass with a remote display especially adapted for automotive vehicles is disclosed in the Dinsmore U.S. Pat. No. 4,402,142 granted Sept. 6, 1983. The compass of this patent is provided with a Hall effect angular position encoder for producing electronic direction data signals. The remote display disclosed in the Dinsmore patent provides a heading read-out which is expressed in abbreviated notation for the compass points such as N, E, S and W for the cardinal points and NE, SE, SW and NW for the intercardinal or secondary compass points.
Another remote indicating magnetic compass for use in an automotive vehicle is disclosed in the Edwards U.S. Pat. No. 4,146,970 granted Apr. 3, 1979. According to this patent, the compass comprises an encoding wheel rotatably mounted within a housing on a pivot pin. A compass magnet is carried on the pivot pin. The encoding wheel is provided with code tracks each of which comprises a segment of light reflective surface and a segment of nonreflective surface. A set of four optical detectors are spaced circumferentially around the wheel to provide a digital signal which represents the angular position of the encoding wheel. The degree of resolution for determining the angular position is dependent upon the number of code tracks on the wheel.
The use of angular position encoders of the optical type in remote indicating compasses is well known. The Fowler U.S. Pat. No. 3,746,842 granted July 17, 1973 discloses a magnetic compass and an optical encoder for producing a digital signal representing an indication of direction. A rotor, in the form of a circular disk, is supported in a frame on a pivot pin which also carries a compass bar magnet. The circular card bears a coded mask and a photodetector generates a digitally coded signal corresponding to the position of the rotor with respect to the frame. The coded mask comprises a plurality of annular tracks of different radius. The tracks constitute a code representing angular position with each track comprising alternate light transparent and opaque sectors. The tracks are coded in a binary code such as the Gray code. In this coding, each individual track contributes one bit of a binary number and hence the degree of resolution of angular position is determined by the number of tracks. A separate photodetector is provided for each track and the set of detectors generates a digital electronic signal corresponding to angular position. Remote reading magnetic compasses utilizing optical position encoders of this type are also described in the Fowler U.S. Pat. No. 4,047,168 granted Sept. 6, 1977 and the Lapeyre U.S. Pat. No. 4,414,754 granted Nov. 15, 1983.
Optical shaft encoders which generate a digital signal representing the angular position of the shaft are known in various forms. A shaft position encoder is known in which a single code track is provided on a code disk and a pair of code element detectors is provided to generate out-of-phase pulses in response to the code elements The extent of angular displacement is determined by counting pulses and the direction of displacement is determined by the phase relationship of the pulses. Such an encoder is shown in the McAuslan et al U.S. Pat. No. 2,944,157 granted July 5, 1960. The Weber U.S. Pat. No. 4,319,134 granted Mar. 9, 1982 discloses an optical encoder for producing an electrical signal representative of the angular position and velocity of a rotatable shaft. This patent describes, as prior art, an optical encoder with a code disk having two annular tracks, one track having uniform distribution of slots and the other track having a single slot indicative of an index or home position. A pair of photodetectors coacting with the one track produce signals which are phase displaced in accordance with the direction of rotation of the shaft. An up/down counter produces an indication of the angular position of the shaft. The Avins U.S. Pat. No. 4,328,463 granted May 4, 1982 discloses an optical encoder for recording incremental changes. This patent describes, as prior art, an encoder in which an encoder disk has a row of apertures with a pair of photodetectors adapted to detect the apertures. The photodetectors are disposed relative to each other so that the signals produced thereby will be out-of-phase with each other. In this system, the digital signals generated by the detectors have four different permutations of logic levels within a single cycle Thus, the direction of rotation can be determined from a comparison of the current signal with the preceding signal.
Although magnetic compasses are well known which utilize angular position encoders for producing electronic direction signals for remote display, such compasses as are known in the prior art are not well adapted for use in automotive vehicles and small boats because of complexity, size and cost. A compass for such vehicles must be adapted for mass production at low cost and yet it must provide an accuracy consistent with the need for the vehicle navigation. The compass must be of rugged construction and reliable in operation and it must lend itself to mounting in a variety of locations in the vehicle. The compass i.e. the combined magnetic field sensor and the angular position encoder, should be of very small size.
A general object of this invention is to provide an improved method and apparatus for finding the heading of a vehicle.